clarify that cell model has parent_model
use _ suffix for private readonly
autosave and save button

displays a highlighted clickable disk button in the navbar
when there is changed content it's possible to save

add styles for .navbar .btn-link
move enable/disable button functions to ui_utils
move ignore_programmatic_changes function to ui_utils
(used for on_dirty as well as command history)

add save button to main.html/main.js

save button and timer logic in notebook controller (?)
new on_dirty event (dishers) from notebook model

save_notebook method for shell_tab

closes #126
artifacts
Merge branch 'develop' of https://github.com/att/rcloud into develop

Conflicts:
	htdocs/js/rcloud_bundle.min.js